1. Field of the Invention
The present invention relates to a microwave radiator for warming therapy, particularly, to a microwave radiator for warming therapy directly inserted into an affected part.
2. Description of the Prior Art
This kind of applicator for warming therapy has been previously developed as is shown in Prior Art FIG. 14 (1) (2) and, Prior Art FIG. 15 (1) (2).
An example of the conventional art, shown in FIG. 14 (1), is of the same kind as a monopole antenna, and is constructed similarly to an applicator in which the outer conductors in the tips of the coaxial cables are removed. More specifically, it has a central conductor 40, and an outer conductor 42 provided around a dielectric 41. The tip of the outer conductor 42 is removed a distance equal to the predetermined length of an A part.
This kind of applicator for warming therapy, as shown in FIG. 14 (2), has the skirt 43 added to it, the base end of which is short-circuited. This prevents a current from flowing on the surface of the conductor of the coaxial cables, thereby stopping overheating of the normal tissues of a human body in the vicinity of the conductor.
The example of a conventional applicator shown in FIG. 15 (1) is characterized in that a band-like conductor 50 is wound helically around the outer surface of the dielectric 41 of the example of the conventional applicator shown in FIG. 14 (1) described above, and thus a helical slit 51 is formed.
The example of the conventional applicator shown in FIG. 15 (1) is disclosed, for example, in Japanese Utility Model Unexamined Publication No. 61-33961 as being a body cavity insertion type. The applicator of this Japanese Utility Model Unexamined Publication No. 61-33961 can be used for inserting into the tissues of a focus and for warming it by making it smaller.
However, the above described example of the conventional applicator has significant disadvantages concerning the function of warming the focuses.
This disadvantage will be concretely described below.
First, in the examples of the conventional applicators shown in FIG. 14 (1) (2) and FIG. 15 (1) (2), the length A of the central conductor protruding from the coaxial cable B is basically a wavelength about 1/4 the frequency in use. More specifically, when the frequency to be used in the muscular tissues of a human body is set to 400 MHz, the following lengths are needed: 2.0 cm at 400 MHz, 5.5 cm at 100 MHz, and 8.5 cm at 60 MHz. When the length is shorter than these, the input impedance matching becomes poor, and radiation efficiency weakens.
On the other hand, in the warming therapy, it is desirable to be able to warm and cure an extensively affected part with one microwave radiator. This is important from the viewpoint of softening the pain of a patient, caused by many microwave radiators being inserted into the body.
By the way, the tissues of the body can be handled as a kind of dielectric-lost material including tissues of a malignant tumor. For this reason, in the case of electromagnetic waves, the lower their frequencies are, the better osmotic permeability they have.
For example, when microwaves are radiated and propagated from the surface of the body to the muscular tissues, the osmotic depth at which the power is reduced by half is as follows: about 1.0 cm at 400 MHz, about 1.4 cm at 200 MHz, about 2.2 cm at 100 MHz, and about 3.0 cm at 60 MHz. This indicates that in the warming therapy in which those applicators shown in FIG. 14 (1) (2) are used, the frequency need be selected depending on the depth from the surface of the body.
Therefore, the selection of the frequency to be used is irrespective of the extent of the affected part in the horizontal direction.
For this reason, for an affected part which is flat and extending in the horizontal direction, an applicator that operates at a high frequency must be used. Since high frequency microwaves have a poor osmotic permeability for tissues, many microwave radiators are needed. This led to inconveniences such that patients suffer much pain from the many insertions.
The applicator of FIG. 15 (1) is, originally, of the body cavity insertion type, and can also be used as an insertion type as described above.
The applicator of FIG. 15 (1) is one in which a consecutive helical slit 51 is provided on the outer conductor 42 of a high-frequency coaxial cable or equivalent, and its tip is short-circuited to the inner conductor 40.
The objective of the microwave radiator shown in FIG. 15 (1) is to warm by means of leak waves from a helical slit 51. However, actually, it does not operate in such a way.
That is, the helical slit 51 provided in a very thin coaxial cable for high frequency as compared with its wavelength results from forming a helical antenna, whose length of one helical winding is very short.
It is generally well known that this helical antenna does hardly match coaxial cable and radiation efficiency is poor.
In this case, the inner conductor of a high-frequency coaxial cable lies at the position of the helical axis, so this structure causes even waves whose phase speed is very close to the light speed (3.times.10.sup.8 m/sec) to propagate. That is, the surface waves propagate. In this structure, the input impedance matching is improved to some extent, as compared with the case of only a very small helical antenna compared with the above described wavelength. The reason is that the presence of the inner conductor serves as a surface wave transmission path. However, this surface wave weakens rapidly away from the axis (the inner conductor) of the transmission path (see FIG. 15 (2)). For this reason, the warming area is limited to the transmission path, i.e., the area most close to the microwave radiator. For this reason, the example of the conventional art shown in FIG. 15 (1) is limited to the applicator having the same effect as that shown in FIG. 14 (1) (2).